U.S. patent number 6,277,306 [Application Number 09/200,457] was granted by the patent office on 2001-08-21 for electro-rheological fluid having high dielectric breakdown stength and methods of making and storing the electro-rheological fluid.
This patent grant is currently assigned to Bridgestone Corporation. Invention is credited to Shigeki Endo, Takayuki Maruyama, Tasuku Saito, Hiroaki Wada.
United States Patent |
6,277,306 |
Endo , et al. |
August 21, 2001 |
Electro-rheological fluid having high dielectric breakdown stength
and methods of making and storing the electro-rheological fluid
Abstract
An electro-rheological fluid made from an oil medium and fine
particulates by mixing, is highly reliable and in which dielectric
breakdown, such as generation of electrical discharge, is very low
even when a high voltage is applied. A method of manufacturing the
electro-rheological fluid and a method of storing the fluid are
also provided. The electro-rheolological fluid includes fine
particulates dispersed in an oil medium having an electric
insulation property, and has a dielectric breakdown strength of 4
kV/mm or more. When the electro-rheological fluid is placed under a
reduced pressure of 10 Pa, foaming does not occur. Alternatively,
the electro-reheoleological fluid can contain 20% by volume or more
of a gas contained in the oil medium that has a dielectric
breakdown strength of 4 kV/mm or more.
Inventors: |
Endo; Shigeki (Tokorozawa,
JP), Maruyama; Takayuki (Kodaira, JP),
Wada; Hiroaki (Kawasaki, JP), Saito; Tasuku
(Tokorozawa, JP) |
Assignee: |
Bridgestone Corporation (Tokyo,
JP)
|
Family
ID: |
18377125 |
Appl.
No.: |
09/200,457 |
Filed: |
November 27, 1998 |
Foreign Application Priority Data
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Dec 15, 1997 [JP] |
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9-345517 |
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Current U.S.
Class: |
252/572; 252/70;
252/73 |
Current CPC
Class: |
C10M
171/001 (20130101) |
Current International
Class: |
C10M
171/00 (20060101); H01B 003/20 () |
Field of
Search: |
;252/572,73 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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5-112793 |
|
May 1993 |
|
JP |
|
11-172269 |
|
Jun 1999 |
|
JP |
|
94/09097 |
|
Apr 1994 |
|
WO |
|
Primary Examiner: Kopec; Mark
Assistant Examiner: Hamlin; D G
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A method of manufacturing an electro-rheological fluid,
comprising stirring and mixing fine particulates and an oil medium
under a reduced pressure of 1 kPa or less.
2. The method of claim 1, wherein the reduced pressure is 0.1 kPa
or less.
3. The method of claim 1, wherein the oil medium comprises 20% or
more by volume of at least one gas selected from the group
consisting of SF.sub.6, CCl.sub.2 F.sub.2, C.sub.3 F.sub.8, C.sub.2
F.sub.6, C.sub.5 F.sub.8, CF.sub.3 CN, C.sub.2 F.sub.5 CN,
Cl.sub.2, SOF.sub.2, C.sub.2 ClF.sub.5, and ClO.sub.3 F.
4. The method of claim 1, wherein the electro-rheological fluid has
a dielectric breakdown strength of at least 4 kV/mm.
5. The method of claim 1, wherein the reduced pressure is 0.01
kPa.
6. A method of manufacturing an electro-rheological fluid,
comprising degassing a mixture obtained by stirring and mixing fine
particulates and an oil medium under a reduced pressure of 11 kPa
or less.
7. The method of claim 6, wherein the degassing comprises heating
the mixture at a temperature of 40 to 80.degree. C. while the
mixture is being degassed.
8. The method of claim 6, wherein the degassing comprises stirring
the mixture while the mixture is being degassed.
9. The method of claim 6, wherein the degassing comprises
irradiating the mixture with ultrasonic waves while the mixture is
being degassed.
10. The method of claim 6, wherein the reduced pressure is 0.1 kPa
or less.
11. The method of claim 6, wherein the electro-rheological fluid
has a dielectric breakdown strength of at least 4 kV/mm.
12. The method of claim 6, wherein the reduced pressure is 0.01
kPa.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-rheological fluid, and
more particularly to an electro-rheological fluid having high
resistance to dielectric breakdown (hereinafter, "dielectric
breakdown strength"), and methods of manufacturing and storing the
same.
2. Description of the Related Art
An electro-rheological fluid is a fluid that can significantly and
reversibly change its rheological characteristics with electrical
control. The phenomenon of dramatic change of the apparent
viscosity of a fluid through the application of an electric field
has been known as the Winslow effect for a long time. The
application of this effect to components for electrically
controlling devices or parts, such as clutches, valves, engine
mounts, actuators, and robot arms has been discussed.
As a result, many proposals have been made on particles or liquid
mediums used as a dispersoid for the purpose of obtaining a fluid
having a high electro-rheological effect and excellent
reproducibility. Conventionally, when a high voltage of 2.5 to 3.5
kV/mm or more is applied to an electro-rheological fluid
(hereinafter occasionally referred to as "ERF") under a flow, it is
found that electrical discharge generates frequently such that the
fluid cannot be actually used.
It has been discovered that an oil type medium, such as
dimethylsilicone or fluorosilicone, and anhydrous particulates such
as carbonaceous particulates which form this ERF have dielectric
breakdown strengths of 6 kV/mm or more respectively. Accordingly,
even if both the oil type medium and the conductive particles used
are materials having high dielectric breakdown strengths, it is
found that the dielectric breakdown strength of the obtained ERF
does not reach a desired level.
When such an electro-rheological fluid is applied to dampers or
clutches, a practical vibration controlling effect or the like
cannot be obtained due to electrical discharge, and thereby
reliability lowers.
An object of the present invention is to provide an
electro-rheological fluid made of an oil type medium and fine
particulates by mixing, which is highly reliable and in which
dielectric breakdown such as generation of electrical discharge
hardly occurs at all even if high voltage is applied to the ERF.
Further, a second object of the present invention is to provide a
method of manufacturing an electro-rheological fluid by which such
an electro-rheological fluid is obtained through a simple process
and a method of storing an electro-rheological fluid which can
prevent reduction in performance of the fluid at the time of
conveyance and storage.
As a result of assiduous studies, the inventors have found that, in
a state in which air or air forming gas (nitrogen, oxygen, argon,
or the like) is included in an ERF made of an oil type medium and
fine particulates by mixing, electrical discharge generates when a
high voltage of 3.5 kv/mm or more is applied under a flow, and that
the above-described objects are achieved by preventing the
generation of electrical discharge. The present invention has been
thereby completed.
SUMMARY OF THE INVENTION
An electro-rheological fluid of the present invention is an
electro-rheological fluid comprising fine particulates dispersed in
an oil type medium having an electric insulation property and has a
dielectric breakdown strength of 4 kV/mm or more. As a preferable
aspect, foaming does not take place when the electro-rheological
fluid is placed under a reduced pressure of 10 Pa, or
alternatively, 20% or more by volume of a gas contained in the oil
type medium is a gas having a dielectric breakdown strength of 4
kV/mm or more.
Further, a method of manufacturing an electro-rheological fluid of
the present invention includes a step of stirring and mixing
particles and an oil type medium under a reduced pressure of 10 kPa
(about 0.1 atmospheric pressure) or less, preferably 1000 Pa or
less, or alternatively, a step of degassing a mixture, which was
obtained by stirring and mixing particles and an oil type medium
and which is disposed under a reduced pressure of 10 kPa or less,
preferably 1000 Pa or less. It is preferable that this degassing
step takes place while heating the mixture at 40.degree. C. to
80.degree. C. and/or stirring the mixture with rotational stirring
blades or with irradiation of supersonic waves, or the like.
In a method of storing an electro-rheological fluid of the present
invention, a container for storing the electro-rheological fluid,
in which fine particulates are dispersed in an oil type medium
having an electric insulation property, is filled with a gas having
a strong electron attracting capability and a high dielectric
breakdown strength and is thereafter sealed. It is preferable that
the gas having a strong electron attracting capability and a high
dielectric breakdown strength is of one or more selected from among
SF.sub.6, CCl.sub.2 F.sub.2, C.sub.3 F.sub.8, C.sub.2 F.sub.6,
C.sub.5 F.sub.8, CF.sub.3 CN, C.sub.2 F.sub.5 CN, Cl.sub.2,
SOF.sub.2, C.sub.2 ClF.sub.5, and ClO.sub.3 F (each having a
halogen atom, a CN group, or a SO group in its molecule).
DETAILED DESCRIPTION OF THE INVENTION
Fine particulates which are preferably used for an
electro-rheological fluid of the present invention can be any
particle that is known as a particle for an electro-rheological
fluid. These include organic semiconductor particulates,
carbonaceous particulates, polyurethane particulates,
surface-insulated membrane coated particulates, organic and
inorganic complex particulates, ceramic particulates, hydrous
particulates, or the like.
A preferable example of the fine particulates which can be used in
the present invention is a carbonaceous particle. As the
carbonaceous particulates, those having a carbon content of 80 to
97% by weight are preferable, and 85 to 95% by weight are more
preferable. Further, a C/H ratio (carbon/hydrogen atom ratio) of
the carbonaceous particles is preferably from 1.2 to 5, and is more
preferably from 2 to 4.
It has been known for a long time that the electrical resistance of
the dispersed phase of an electro-rheological fluid is, in general,
in a semiconductor domain (W. M. Winslow: J. Appl. Physics vol. 20,
page 1137 (1949)), however, carbonaceous particles having a carbon
content of less than 80% by weight and a C/H ratio of less than 1.2
are insulating materials, and thus a fluid having an
electro-rheological effect can barely be obtained therefrom. On the
other hand, those having a carbon content of more than 97% by
weight and a C/H ratio of more than 5 are similar to conductive
materials and show an excessively large electric current even when
voltage is applied, and thus a fluid having an electro-rheological
effect cannot be obtained.
Examples of methods of manufacturing these carbonaceous particles
include a method of heat-treating pulverizing, and classifying
mesophase that was produced by heat-treating pitch or the like, a
method of carbonizing a thermosetting resin by heat-treating, and a
method of carbonizing aromatic sulfonic acids or a condensation
product of a salt thereof, formed in a fine spherical body, by
heat-treating in an inert gas atmosphere such as nitrogen, argon,
or the like.
The average particle size of the particles can be measured with a
particle size analyzer (e.g., MICROTRAC SPA/MK-II TYPE manufactured
by Nikldso Co., Ltd.) as mentioned in the Examples. The average
particle size of the particles for an electro-rheological fluid
obtained after the carbonizing treatment is preferably about 0.1 to
20 .mu.m, and more preferably 0.5 to 15 .mu.m. If the average
particle size is less than 0.1 .mu.m, the initial viscosity of the
obtained electro-rheological fluid becomes high. If the average
particle size is more than 20 .mu.m, the dispersion stability of
the particles deteriorates. Neither is preferable.
The electro-rheological fluid of the present invention is obtained
by dispersing the particles for the electro-rheological fluid
obtained as described above in an oil type medium having an
electric insulation property. The particles for an
electro-rheological fluid, which are dispersed phase, are contained
in the electro-rheological fluid at a level of 1 to 60% by weight,
preferably 20 to 50% by weight, and the oil type medium, which is
the dispersion medium, is contained at a level of 99 to 40% by
weight, preferably 80 to 50% by weight. If the dispersed phase
content is less than 1% by weight, the electro-rheological effect
is small, and if the dispersed phase content is more than 60% by
weight, the initial viscosity when voltage is not applied becomes
high, and thus neither is preferable.
The oil type medium, which is a dispersion medium having an
electric insulation property, preferably has a volume resistivity
at 80.degree. C. of 10.sup.11.OMEGA..cm or more. A value of
10.sup.13.OMEGA..cm or more is particularly preferable. For
example, hydrocarbon oil, ester type oil, aromatic type oil, and
silicone oil can be presented. Concrete examples include an ester
of an aliphatic monocarboxylic acid such as neocapric acid; an
ester of an aromatic monocarboxylic acid such as benzoic acid; an
ester of an aliphatic dicarboxylic acid such as adipic acid,
glutaric acid, sebacic acid, or azelaic acid; an ester of an
aromatic dicarboxylic acid such as phthalic acid, isophthalic acid,
or tetrahydrophthalic acid; dimethyl polysiloxane, methyl phenyl
polysiloxane, methyl trifluoro propyl polysiloxane, and a mixture
or copolymer of these. These can be used alone or in combinations
of two or more.
The oil type medium having an electric insulation property
preferably has a viscosity at 25.degree. C. of 0.65 to 500
centistokes, more preferably 2 to 200 centistokes. A value of 5 to
50 centistokes is particularly preferable. By using a dispersion
medium having a preferable viscosity, the particles, which are
dispersoid, can be dispersed efficiently and stably. If the
viscosity of the oil type medium is more than 500 centistokes, the
initial viscosity of the electro-rheological fluid becomes high,
resulting in a small viscosity change brought about by the
electro-rheological effect. Further, if the viscosity is less than
0.65 centistokes, evaporation easily occurs, and the stability of
the dispersion medium deteriorates.
The electro-rheological fluid of the present invention needs to
have a dielectric breakdown strength of 4 kV/mm or more. However,
in order to achieve this, it is important to prevent the
aforementioned air or gas (nitrogen, oxygen, argon, or the like)
which forms air from being included in the fluid. In order to
verify this characteristic, first, the electro-rheological fluid is
placed under a reduced pressure of 10 Pa. If foaming does not take
place, it is preferable. In this way, a gas component is hardly
included in the electro-rheological fluid in a flowing state. If
foaming does not take place in this state, it is considered that
inclusion of the gas does not affect reduction in dielectric
breakdown strength.
On the other hand, inclusion of a gas becomes a problem only when a
gas that causes electrical discharge when high voltage is applied
to the electro-rheological fluid in a foaming state is included,
and it is not a problem when a gas that does not have such a
characteristic is included. More specifically, if 20% or more by
volume or more of the gas contained within the oil type medium of
the electro-rheological fluid of the present invention has a
relatively large molecular weight and a high dielectric breakdown
strength, in other words, if the gas has a strong electron
attracting capability and a dielectric breakdown strength of 4
kV/mm or more, reduction in the dielectric breakdown strength of
the ERF does not occur. The dielectric breakdown strength of gas
can be measured in accordance with an ordinary method.
Concrete examples of the gas that does not reduce these dielectric
strengths, i.e., the gas that has a dielectric breakdown strength
of 4 kV/mm or more, include SF.sub.6 (hereinafter, the dielectric
breakdown strength will be referred in a parenthesis: 6.6 kV/mm),
CCl.sub.2 F.sub.2 (6.4 kV/mm), C.sub.3 F.sub.8 (5.8 kV/mm), C.sub.2
F.sub.6 (4.8 kV/mm), C.sub.5 F.sub.8 (14.5 kV/mm), CF.sub.3 CN (9.2
kV/mm), C.sub.2 F.sub.5 CN (11.9 kV/mm), Cl.sub.2 (4.1 kV/mm),
SOF.sub.2 (6.6 kV/mm), C.sub.2 ClF.sub.5 (6.0 kV/mm), and ClO.sub.3
F (7.2 kV/mm). Each has a halogen atom, a CN group, or an SO group
in its molecule.
Next, methods of manufacturing the electro-rheological fluid of the
present invention will be explained. Examples of the methods of
manufacturing the electro-rheological fluid include a method of
manufacturing the ERF by mixing particles and an oil type medium
under reduced pressure and a post-treatment that efficiently
deaerates air or air-forming gas, under reduced pressure, from an
ERF that has been mixed under normal pressure. The dielectric
strength of the ERF is remarkably improved in accordance with
either of these two methods.
Namely, the former method includes a process for stirring and
mixing the particles and the oil type medium under reduced
pressure. The reduced pressure is 10 kPa (about 0.1 atmospheric
pressure) or less, preferably 1000 Pa or less, and more preferably
100 Pa or less.
On the other hand, when the electro-rheological fluid that has been
manufactured at a normal pressure is subjected to reduced pressure
and degassing, a mixture obtained by stirring and mixing the
particles and the oil type medium is placed under reduced pressure
and subjected to degassing for a predetermined time. At this time,
it is preferable that degassing is effected while the mixture is
heated at 40.degree. C. to 80.degree. C. and/or stirred. This
condition of reduced pressure is the same as the one at the time of
manufacturing.
Moreover, a condition of heating is preferably within a range of 40
to 80.degree. C. If the temperature is less than 40.degree. C.,
viscosity of the fluid is high and degassing is not carried out
sufficiently. If the temperature is more than 80.degree. C., there
is worry that stability of the electro-rheological fluid
deteriorates.
The stirring in the process of degassing can be effected in
accordance with an ordinary method. For example, the stirring may
be effected with rotational stirring blades or with irradiation of
ultrasonic a waves. In a case of rotational stirring, the
rotational speed of the blades is preferably about 10 to 200 rpm.
In a case of ultrasonic irradiation, the output is preferably 30 W
or more.
On the other hand, when the above-described (mixed under reduced
pressure or degassing) ERF having a high dielectric breakdown
strength is used/stored, there is worry that gas may be included
again and the dielectric strength may be reduced when, for example,
the ERF is vibrated within a container during conveyance. Thus, a
gas having a strong electron attracting capability and a high
dielectric breakdown strength rather than air or air-forming gas is
included together with the ERF in a device or a storage container.
Accordingly, even if the device is operated or the entire storage
container is vibrated, reduction in the dielectric strength of the
ERF due to inclusion of gas into the fluid can be prevented.
It is considered that the gas having a high dielectric breakdown
strength has, in general, a large electronegativity (more
specifically, a dielectric breakdown strength of 4.0 kV/mm or more)
and a large molecular weight. Examples of the gas include SF.sub.6,
CCl.sub.2 F.sub.2, C.sub.3 F.sub.8, C.sub.2 F.sub.6, C.sub.5
F.sub.8, CF.sub.3 CN, C.sub.2 F.sub.5 CN, Cl.sub.2, SOF.sub.2,
C.sub.2 ClF.sub.5, and ClO.sub.3 F, each having a halogen atom, a
CN group, or a SO group in its molecules.
A high dielectric strength at the time of manufacturing can be
maintained by conveying the ERF in a container filled with such a
gas. Further, reduction in the dielectric strength with time can be
prevented and high reliability can be maintained for a long time by
filling such a gas in a device such as a damper, which is used by
being filled with an electro-rheological fluid.
EXAMPLES
Hereinafter the present invention will be explained in more detail
with reference to concrete examples. However, the present invention
is not limited to these examples.
Property Evaluations
(1) Measurement of Particle Size
The particle size of the particles for the electro-rheological
fluid was measured with a MICROTRAC SPA/MK-II type manufactured by
Nikkiso Co., Ltd.
(2) Measurement of Dielectric Breakdown Strength of ERF
When the field strength was increased from 3.0 kV/mm at an interval
of 0.1 kV/mm every 30 seconds at room temperature (about 25.degree.
C.) and at a shearing rate of 1000/second with an RDS-II type
rheometer manufactured by RHEOMETRICS Co., Ltd. and a 610 type high
voltage power supply manufactured by TREK Co., Ltd., the field
strength at which electrical discharge was generated was referred
to as the dielectric breakdown strength (dielectric strength) of
ERF.
In this case, since high voltage was applied for 10 minutes until
the field strength reaches, for example, 5.0 kV/mm, the dielectric
breakdown strength of the ERF obtained in this method is estimated
to be lower than the actual characteristic value of the material
(in other words, the actual dielectric breakdown strength is much
higher).
Example 1
Preparation of Carbonaceous Particle Material
1050 g of sulfuric acid was added to 1280 g of naphthalene, and
reacted at 160.degree. C. for 2 hours. An unreacted product was
discharged outside container under reduced pressure. Then, 857 g of
35% by weight concentration formalin was added and reacted at
105.degree. C. for 5 hours to obtain a methylene bond type
condensation product of .beta.-naphthalene sulfonic acid. After
neutralization with ammonium water, the condensation product was
filtrated to yield a filtrate.
Water was added to the obtained filtrate containing a methylene
bond type condensation product of .beta.-naphthalene sulfonic acid
so as to prepare a 20% by weight concentration aqueous solution of
a methylene bond type product of ammonium salt of
.beta.-naphthalene sulfonate.
This aqueous solution was sprayed with a spray drier at an air
pressure of 5 kg/cm.sup.2 and granulated/dried by introducing
drying air. The average particle size (50% volume average size) of
the spherical carbonaceous particles of the methylene bond type
condensation product of sulfonic acid mainly comprising methyl
naphthalene obtained as mentioned above was 7.0 .mu.m.
Preparation of Particles for Electro-Rheological Fluid
Spherical particles were obtained by a preliminary heat treatment
of the obtained carbonaceous particles at 400.degree. C. in a
nitrogen gas atmosphere. The carbon content, the carbon/hydrogen
atom ratio (hereinafter referred to as the C/H ratio), and the
average particle size of the particles were 90.8%, 2.0, and 7.0 gm,
respectively. The spherical particles for the electro-rheological
fluid were obtained by heating (carbonizing treatment) at
540.degree. C. in a nitrogen gas atmosphere for 4 hours, atomizing,
and classifying. The carbon content, the C/H ratio, and the average
particle size of the particles were 93.6%, 2.4, and 7 .mu.m,
respectively.
Preparation of Electro-Rheological Fluid
45% by weight of the spherical carbonaceous particles obtained in
Example 1 was dispersed well in 55% by weight of a copolymer of
dimethyl polysiloxane and methyl trifluoro propyl polysiloxane (mol
ratio of 60 to 40), which is a dispersing medium having a viscosity
of 10 centistokes at 25.degree. C., and was stirred under reduced
pressure of 1000 Pa so as to obtain an electro-rheological
fluid.
The dielectric strength of the electro-rheological fluid obtained
was measured in accordance with the above method, and as a result,
the dielectric strength was found to be 4.2 kV/mm.
Example 2
Under the same conditions as Example 1 except that the reduced
pressure was 100 Pa, an electro-rheological fluid was obtained. The
dielectric strength of the electro-rheological fluid obtained was
measured in accordance with the same method as Example 1, and as a
result, it was found that electrical discharge did not occur at
least at 4.5 kV/mm and that the dielectric strength was 4.5 kV/mm
or more.
Example 3
Under the same conditions as Example 2 except that the reduced
pressure was 10 Pa, an electro-rheological fluid was obtained. The
dielectric strength of the electro-rheological fluid obtained was
measured in accordance with the same method as Example 1, and as a
result, it was found that electrical discharge did not occur at
least at 5.1 kV/mm and that the dielectric strength was 5.1 kV/mm
or more.
Comparative Example 1
Under the same conditions as Example 1 except that pressure was not
reduced and stirring was effected with a ball mill under normal
pressure, an electro-rheological fluid was obtained. The dielectric
strength of the electro-rheological fluid obtained was measured in
accordance with the same method as in Example 1, and as a result,
the dielectric strength was found to be 3.2 kV/mm.
Comparative Example 2
Under the same conditions as Example 1 except that pressure was not
reduced and stirring was effected with a mortar grinder under
normal pressure, an electro-rheological fluid was obtained. The
dielectric strength of the electro-rheological fluid obtained was
measured in accordance with the same method as Example 1, and as a
result, the dielectric strength was found to be 3.6 kV/mm.
Example 4
The electro-rheological fluid obtained in Comparative Example 2 was
held under a reduced pressure of 10 Pa for 30 minutes in a sealable
container and subjected to a degassing treatment. The dielectric
strength of the treated electro-rheological fluid was measured in
accordance with the same method as Example 1, and as a result, the
dielectric strength was found to be 4.2 kV/mm.
Example 5
The electro-rheological fluid obtained in Comparative Example 2 was
held under a reduced pressure of 10 Pa for 30 minutes in a vacuum
chamber while being heated at 60.degree. C. and subjected to a
degassing treatment. The dielectric strength of the
electro-rheological fluid treated was measured in accordance with
the same method as Example 1, and as a result, it was found that
electrical discharge did not occur at least at 5.0 kV/mm and that
the dielectric strength was 5.0 kV/mm or more.
Example 6
The electro-rheological fluid obtained in Comparative Example 2 was
stirred by rotational blades under a reduced pressure of 10 Pa for
60 minutes in a vacuum chamber and subjected to a degassing
treatment. The rotational speed of the stirring blades was 40 rpm.
The dielectric strength of the treated electro-rheological fluid
was measured in accordance with the same method as Example 1, and
as a result, it was found that electrical discharge did not occur
at least at 5.0 kV/mm and that the dielectric strength was 5.0
kV/mm or more.
Example 7
The electro-rheological fluid obtained in Comparative Example 2 was
stirred by irradiating ultrasonic waves under a reduced pressure of
10 Pa for 15 minutes in a vacuum chamber and subjected to a
degassing treatment. The conditions of irradiation of the
ultrasonic waves was 40 Hz and 100 W. The dielectric strength of
the treated electro-rheological fluid was measured in accordance
with the same method as Example 1, and as a result, it was found
that electrical discharge did not occur at least at 5.0 kV/mm and
that the dielectric strength was 5.0 kV/mm or more.
Comparative Example 3
Under the same conditions as Comparative Example 2 except that
dimethyl polysiloxane having a viscosity of 10 centistokes at
25.degree. C. (TSF451-10 manufactured by Toshiba Silicone Co.,
Ltd.) was used as the oil type medium, an electro-rheological fluid
was obtained. The dielectric strength of the electro-rheological
fluid obtained was measured in accordance with the same method as
Example 1, and as a result, the dielectric strength was found to be
3.9 kV/mm.
Example 8
The electro-rheological fluid obtained in Comparative Example 3 was
subjected to a degassing treatment in a vacuum chamber, while being
heated at 60.degree. C., under a reduced pressure of 10 Pa for 30
minutes. The dielectric strength of the electro-rheological fluid
treated was measured in accordance with the same method as Example
1, and as a result, it was found that electrical discharge did not
occur at least at 5.0 kV/mm and that the dielectric strength was
5.0 kV/mm or more.
Comparative Example 4
Under the same conditions as Example 1 except that an operating
condition of the spray drier was changed in the processes. The
particles were classified by a classifier such that particles for
an electro-rheological fluid were obtained. The carbon content, the
C/H ratio, and the average particle size of the particles were
93.5%, 2.2, and 3 .mu.m, respectively.
An electro-rheological fluid was obtained in the same way as
Comparative Example 2, using these particles for an
electro-rheological fluid. The dielectric strength of the
electro-rheological fluid obtained was measured in accordance with
the same method as Example 1, and as a result, the dielectric
strength was found to be 3.8 kV/mm.
Example 9
The electro-rheological fluid obtained in Comparative Example 4 was
subjected to a degassing treatment in a vacuum chamber, while being
heated at 60.degree. C., under a reduced pressure of 10 Pa for 30
minutes. The dielectric strength of the treated electro-rheological
fluid was measured in accordance with the same method as Example 1,
and as a result, it was found that electrical discharge did not
occur at least at 5.0 kV/mm and that the dielectric strength was
5.0 kV/mm or more.
Comparative Example 5
After a mesophase-growing process by a heat treatment of coal tar
pitch at 450.degree. C. in a nitrogen gas atmosphere, the coal tar
pitch was repeatedly extracted and separated by filtration in tar
oil to eliminate the pitch component. After another heat treatment
at 350.degree. C. in a nitrogen reflux, it was pulverized to obtain
amorphous particles. The carbon content and the C/H ratio of the
particles were 90.8% and 2.0, respectively. Further, particles for
an electro-rheological fluid were obtained by conducting a heat
treatment at a temperature of 500.degree. C. for 4 hours in a
rotary kiln in a nitrogen atmosphere. The carbon content and the
C/H ratio of the particles were 93.6% and 2.4, respectively.
Using the carbonaceous particles obtained, an electro-rheological
fluid was obtained in the same way as Comparative Example 2. The
dielectric strength of the electro-rheological fluid obtained was
measured in accordance with the same method as Example 1, and as a
result, the dielectric strength was found to be 3.9 kV/mm.
Example 10
The electro-rheological fluid obtained in Comparative Example 5 was
subjected to a degassing treatment in a sealable container, while
being heated at 60.degree. C., under a reduced pressure of 10 Pa
for 30 minutes. The dielectric strength of the electro-rheological
fluid treated was measured in accordance with the same method as
Example 1, and as a result, it was found that electrical discharge
did not occur at least at 4.5 kV/mm and that the dielectric
strength was 4.5 kV/mm or more.
Example 11
Storage of Electro-Rheological Fluid
The electro-rheological fluid obtained in Example 1 was placed in a
sealable storage container. The air within the container was
replaced with SF.sub.6 gas, and thereafter the container was
sealed. This storage container was shaken hard for 10 minutes. The
electro-rheological fluid was removed from the container after
shaking. The dielectric strength of the electro-rheological fluid
processed was measured in accordance with the same method as
Example 1, and as a result, it was found that electrical discharge
did not occur at least at 5.1 kV/mm and that the dielectric
strength was 5.1 kV/mm or more.
Comparative Example 6
Storage of Electro-Rheological Fluid
A storage container was filled with the electro-rheological fluid
obtained in Example 1 together with air and then the container was
sealed. This storage container was shaken hard for 10 minutes. The
electro-rheological fluid was removed from the container after
shaking. The dielectric strength of the electro-rheological fluid
processed was measured in accordance with the same method as
Example 1, and as a result, the dielectric strength was found to be
3.7 kV/mm.
Post Treatment
The electro-rheological fluid stored in Comparative Example 6 was
subjected to a degassing treatment under a reduced pressure of 10
Pa for 30 minutes in a vacuum chamber. The dielectric strength of
the electro-rheological fluid treated was measured in accordance
with the same method as Example 1, and as a result, the dielectric
strength was found to be 4.8 kV/mm. Even if the dielectric strength
of the electro-rheological fluid was reduced by including air
during storage, it was seen that the dielectric strength was
recovered by effecting the post treatment, i.e., the process for
degassing.
Comparative Example 7
Storage of Electro-Rheological Fluid
The electro-rheological fluid obtained in Example 1 was placed in a
sealable storage container. The air within the container was
replaced with Ar gas, and thereafter the container was sealed. This
storage container was shaken hard for 10 minutes. The
electro-rheological fluid was removed from the container after
shaking. The dielectric strength of the electro-rheological fluid
treated was measured in accordance with the same method as Example
1, and as a result, the dielectric strength was found to be 3.9
kV/mm. Accordingly, if the container was filled with inert gas such
as Ar gas or the like, it was found that the reduction in the
dielectric strength of the electro-rheological fluid during storage
cannot be prevented.
Post Treatment
The electro-rheological fluid stored in Comparative Example 7 was
subjected to a degassing treatment under a reduced pressure of 10
Pa for 30 minutes in a vacuum chamber. The dielectric strength of
the treated electro-rheological fluid was measured in accordance
with the same method as Example 1, and as a result, the dielectric
strength was found to be 4.5 kV/mm or more. Even if the dielectric
strength of the electro-rheological fluid was reduced by including
inert gas such as Ar gas or the like during storage, it was seen
that the dielectric strength was recovered by effecting the post
treatment, i.e., the process of degassing.
Example 12
Electro-Rheological Fluid Applied Damper
When the electro-rheological fluid obtained in Example 1 was used
with a damper, the damper was filled with SF.sub.6 gas as a buffer
gas. This damper has excellent dielectric strength in that
electrical discharge does not occur even if the field strength was
applied at 6.0 kV/mm. Further, the same evaluations were made after
this damper was repeatedly used for 56 hours, and it was found that
the dielectric strength was not reduced and that the damper was
highly reliable.
A superior effect is achieved in that the electro-rheological fluid
of the present invention is highly reliable and dielectric
breakdown such as generation of electrical discharge or the like
hardly occurs at all even if a high voltage is applied. Moreover,
the electro-rheological fluid having the above-described excellent
characteristics can be obtained through a simple process in
accordance with the method of manufacturing the electro-rheological
fluid, and reduction in the dielectric strength of the
electro-rheological fluid at the time of conveyance and storage can
be effectively prevented in accordance with the method of storing
the electro-rheological fluid of the present invention.
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